This invention relates to a control system for or in association with a heater (herein referred to as being of the kind specified) comprising an electrical heating means, and a mass of material, herein called the store, in thermal communication with the heating means.
The primary application of the invention is to space heaters and in this case such heater additionally includes means for establishing the flow of a heat distributing fluid for transferring heat from the store to the place to be heated. When the store comprises a body of solid state material or a liquid material which remains in a container the heat distributing fluid may be air which is brought into contact with the store and thereafter caused or permitted to flow to a place at which heat is required to be furnished. Some forms of space heater include water as the heat store medium and in such cases this water may also act as the heat distributing fluid by being caused or permitted to circulate through radiators in the space to be heated.
The invention may, however, be applied to water heaters for producing a supply of heated water for consumption. In this case the store comprises the quantity of water which is stored temporarily in a tank, usually enclosed partly or wholly by thermal insulating material, the heated water being conveyed in pipes to one or more points of consumption as required.
Energisation of the store by its electrical heating means is normally confined to current drawn from a source which is rendered live for one or more limited periods of time during a basic time interval of 24 hours. Such source is normally rendered live during periods when the demand on a mains electrical supply as a whole is lower than at other times, such limited periods being called, and herein referred to as, off-peak periods. Output terminals of such a source are rendered live by closure of contacts automatically, and not under the control of the user, for example by being enclosed in a locked or sealed housing such as a supply meter housing to which only the generating company or organisation has access.
One of such off-peak periods which is frequently utilised extends from 23.00 to 07.00 hours and in some cases a second off-peak period is provided extending from 13.00 to 15.00 hours.
Although the overall load on the mains electricity supply system is almost invariably lower during at least the main night-time off-peak period, that is from 23.00 to 07.00 hours, this overall load can still be quite high during the early part of the night-time off-peak period and in fact typically does not reduce to a level of about 50% of the normal day-time peak load until 03.00 hours or thereabouts.